A mixture of air and fuel is burned in an engine combustion chamber for producing power of an automobile. Herein controlling a ratio of air and fuel (air-fuel ratio) is an important factor for appropriately driving the automobile in accordance with conditions thereof. Inappropriate control of this air-fuel ratio causes deterioration of fuel economy and increase of exhaust gases. In addition, knocking possibly occurs due to abnormal combustion of a mixture. Therefore, a flow sensor is generally disposed in an intake manifold of an engine for detecting a flow quantity of air, thus determining an injection quantity of fuel in accordance with the detected intake air quantity. That is, an air-fuel ratio of a mixture introduced into an engine combustion chamber is controlled.
Air flowing in the intake manifold is supposed to flow in one direction under normal conditions. However, as an engine rotational speed rises from a low speed region to an intermediate region to increase an intake air quantity and an exhaust gas quantity, opening/closing of an intake valve and an exhaust valve overlaps, possibly blowing a part of the exhaust gas back into the intake manifold. As a result, this causes a possible reverse flow of the air in the intake manifold. On such occasion, when a flow sensor, which is incapable of recognizing a flowing direction of the air, is used, the flow sensor has no other choice but to determine that the air flows in a forward direction. Accordingly, it is difficult to accurately control an air-fuel ratio of a mixture. Therefore, in recent years, a flow sensor used particularly for engine control in an automobile is designed to detect a flow quantity of air, as well as recognize whether the air flows in a forward direction or in a reverse direction.
JP-A-8-43162 discloses a thermal-type flow sensor, which is also capable of detecting a reverse direction of air. FIG. 4A shows a circuit diagram showing a circuit arrangement of a flow sensor arranged based upon the technology described in this prior art, and FIG. 4B shows heat generating resistors (Rh5 and Rh6) and thermometer (heat sensing) resistors (Rt2 and Rt3) formed on a substrate 1c in the flow sensor. As shown in FIG. 4B, the flow sensor is formed in such a way that the two heat generating resistors Rh5 and Rh6 are placed to be opposed with each other in the flow direction on the substrate 1c and the thermometer resistors Rt2 and Rt3 respectively for adjusting a temperature of each heat generating resistor are formed on the same substrate 1c. 
These resistors are connected as shown in FIG. 4A. A drive circuit 100 controls a temperature of the heat generating resistor Rh5 so as to be by a predetermined value higher than the surrounding temperature thereof and a drive circuit 200 controls a temperature of the heat generating resistor Rh6 so as to be by a predetermined value higher than the surrounding temperature thereof. In order to control the temperatures of the heat generating resistors Rh5 and Rh6 so as to be the same, each of the drive circuit 100 and the drive circuit 200 is formed of an element having the same properties.
Each of the heat generating resistors Rh5 and Rh6 and the thermometer resistors Rt2 and Rt3 has resistance having temperature-dependent properties, i.e. the resistance, a resistance value of which changes with its temperature and is formed of, for example, a platinum film or a polysilicon film. Since the heat generating resistors Rh5 and Rh6 are controlled to be at high temperatures by the current flowing through themselves, the resistance value of each is made smaller than that of each thermometer resistor Rt2 and Rt3 for smoother flow of the current therethrough.
In the above arrangement, the substrate 1c on which the heat generating resistors Rh5 and Rh6 and the thermometer resistors Rt2 and Rt3 are formed is located in the air fluid so that each resistor is positioned to be perpendicular to a flow direction of air. On this occasion, one of the heat generating resistors Rh5 and Rh6 is positioned in the upstream side with respect to the flow direction of air and the other is positioned in the downstream side with respect thereto.
In addition, each of the heat generating resistors Rh5 and Rh6 is controlled by the respective drive circuits 100 and 200 operated with a power supply voltage Vb in such a way that a temperature of each is higher by a predetermined value (for example, 150° C. ) than the surrounding temperature thereof.
When air flows in the forward direction under such conditions, air cools the heat generating resistor Rh5, thereby reducing a resistance value thereof. A reduction quantity of the resistance value increases in proportion to the air flow. On the other hand, the air heated by the heat generating resistor Rh5 passes through the heat generating resistor Rh6 located in the downstream side of the heat generating resistor Rh5, and therefore, it is not so much cooled. That is, the resistance value of the heat generating resistor Rh6 is not so much reduced as that of the heat generating resistor Rh5. As a result, an electric potential V10 of a terminal 70 to which one end of the heat generating resistor Rh5 is connected increases in comparison with that before the air flows therethrough. On the other hand, an electric potential V20 of a terminal 71 to which one end of the heat generating resistor Rh6 is connected does not change so much before and after the air flows therethrough.
On the contrary, when the air flows in the reverse direction, the resistance value of the heat generating resistor Rh6 reduces and the resistance value of the heat generating resistor Rh5 does not change so much. That is, the electric potential V20 in the terminal 71 increases in comparison with that before the air flows therethrough. The electric potential V10 in the terminal 70 does not change so much before and after the air flows therethrough.
Accordingly, when a difference Vo between the electric potential V10 in the terminal 70 and the electric potential V20 in the terminal 71 is monitored and as a result the difference Vo is a positive value, it is determined that the air flows in the forward direction. When the difference is a negative value, it is determined that the air flows in the reverse direction. Further, a flow speed of the air is calculated on the basis of a magnitude of the difference Vo.
In the above arrangement, the flow sensor for controlling each of the heat generating resistors Rh5 and Rh6 to be at a predetermined temperature is provided with the drive circuits 100 and 200 separately arranged. It is preferable to use the drive circuits 100 and 200, each of which is formed of an element having as much the same properties as possible for controlling temperatures of the heat generating resistors Rh5 and Rh6 in the same way. In particular, in regard to each of differential amplifiers 30 and 31, which monitors an intermediate electric potential difference of each bridge circuit, since each current value flowing in the heat generating resistors Rh5 and Rh6 is determined by each output of the differential amplifiers 30 and 31, it is preferable to use an ideal differential amplifier, an offset voltage of which is zero.
However, variations in each of the differential amplifiers occur, and therefore, it is difficult to provide the differential amplifiers, each having the exactly same properties. Accordingly, the offset voltage is in fact produced in each of the differential amplifiers 30 and 31, and also the offset voltage itself differs in magnitude therebetween. Therefore, the difference in properties between the differential amplifier 30 and the differential amplifier 31 causes an error in temperature control between the heat generating resistor Rh5 and the heat generating resistor Rh6. As a result, the sensitivity to the air flow differs between the heat generating resistor Rh5 and the heat generating resistor Rh6. That is, when the air flow is designed to be more accurately calculated, unevenness in properties between the drive circuit 100 and the drive circuit 200 causes obstruction to accurate calculation. In particular, in recent years, a flow sensor is desired more and more to detect air flow quantity with high accuracy in an internal combustion engine to meet exhaust gas emission regulations for an automobile.